diff --git a/src/lib/png.c b/src/lib/png.c
new file mode 100644
index 000000000..6a5a65380
--- /dev/null
+++ b/src/lib/png.c
@@ -0,0 +1,43 @@
+#include <sys/mman.h>
+#include "visual-details.h"
+#include "internal.h"
+#include "png.h"
+
+size_t compute_png_size(const ncvisual* ncv){
+  return 0;
+}
+
+// write a PNG at the provided buffer using the ncvisual
+int create_png(const ncvisual* ncv, void* buf, size_t* bsize){
+  // FIXME fill it in
+  return -1;
+}
+
+static inline size_t
+mmap_round_size(size_t s){
+  const size_t pgsize = 4096; // FIXME get page size, round up s
+  return (s + pgsize - 1) / pgsize * pgsize;
+}
+
+// write a PNG, creating the buffer ourselves. it must be munmapped. the
+// resulting length is written to *bsize on success. returns MMAP_FAILED
+// on a failure.
+void* create_png_mmap(const ncvisual* ncv, size_t* bsize){
+  *bsize = compute_png_size(ncv);
+  *bsize = mmap_round_size(*bsize);
+  if(*bsize == 0){
+    return MAP_FAILED;
+  }
+  // FIXME hugetlb?
+  void* map = mmap(NULL, *bsize, PROT_WRITE|PROT_READ,
+                   MAP_SHARED_VALIDATE|MAP_ANONYMOUS, -1, 0);
+  if(map == MAP_FAILED){
+    logerror("Couldn't get %zuB map\n", *bsize);
+    return MAP_FAILED;
+  }
+  if(create_png(ncv, map, bsize)){
+    munmap(map, *bsize);
+    return MAP_FAILED;
+  }
+  return map;
+}